The present invention relates to generally an exhaust gas purifying device for use with an internal combustion engine and more particularly an improvement of an exhaust gas recirculation system.
In the conventional exhaust gas recirculation systems, a part of exhaust gas is recirculated from an exhaust system into an intake system downstream of a venturi of a carburetor but upstream of a throttle valve through a fixed restriction in a volume which is a function of the back pressure in the exhaust system. In the conventional secondary air supply systems, the air discharged from an air pump is injected into the exhaust pipe through an air bypass valve for discharging the secondary air in case of deceleration, a relief valve for discharging the secondary air into the surrounding atmosphere when the discharge pressure of the air pump is high in order to improve the service life of the air pump and a check valve for preventing the flow of exhaust gas into the air pump. Under the normal conditions, all of the air discharged from the air pump is injected into the exhaust pipe. The exhaust gas recirculation system and the secondary air supply system are controlled independently of each other.
With the secondary air supply systems of the type described, the air discharged from the air pump is all injected into the exhaust pipe under normal conditions so that the volume of secondary air supply relative to the volume of intake air increases and decreases under light and heavy load. Since the ratio of the volume of secondary air supply to the volume of intake air varies depending upon load, the back pressure is deviated from the relation that the back pressure is proportional to the square of the volume of intake air. Furthermore the back pressure varies not only with load but also with the decrease in performance of the air pump. If the secondary air supply system is not incorporated, the relation between the negative pressure at a venturi in a carburetor which pressure is a function of the volume of intake air and the back pressure in the exhaust system is linear. Therefore when a part of exhaust gas is recirculated from the exhaust system into the intake system downstream of the restriction but upstream of the throttle valve, the volume of exhaust gas recirculated is in proportion to the volume of intake air. However, when the secondary air supply system including the air pump driven by the engine is incorporated, the linear relation between the back pressure and the negative pressure at the venturi in the carburetor is lost and consequently the volume of exhaust gas to be recirculated is not in proportion to the volume of intake air because the volume of secondary air supply is in proportion to the rotational speed of the engine whereas the volume of intake air varies with the variation not only in rotational speed of the engine but also in load.
As a result, in the conventional exhaust gas purifying device incorporating both the exhaust gas recirculation system and the secondary air supply system, the volume of exhaust gas to be recirculated is not in proportion to the volume of intake air. As a consequence, with light load the ratio of the volume of exhaust gas to be recirculated to the volume of intake air increases excessively so that the engine operation is adversely affected. With heavy load the ratio decreases excessively so that the satisfactory elimination of NOx cannot be attained. Furthermore the exhaust gas to be recirculated contains the secondary air injected into the exhaust system so that the air-fuel ratio which is set by the carburetor changes.